The present invention relates to an injection syringe of the type comprising, on the one hand, an elongate syringe body comprising a tube and a perforated front wall capable of receiving an injection needle and, on the other hand, a rear actuating plunger mounted so that it can move inside the tube, which syringe further comprises a mobile needle shield that can move externally along the syringe body between a retracted position drawn back from the injection end of the needle and an active protecting position in which the front end of the shield is forward of the injection end of said needle, there also being means of axial coupling between the shield and the actuating plunger to cause them to move simultaneously forward along a common portion.
A syringe of the aforementioned type is described, for example, in international patent application WO-97/02855 filed in the name of the applicant company.
In the syringe described in that document, the needle protector or shield consists of a tube that can be moved externally along the syringe body. The tube is fixed permanently to the rear end of the actuating plunger. This end is equipped with a disk on which a finger can press when the syringe is actuated.
Thus, the actuating plunger and the needle shield can be moved simultaneously and cover travels of the same length.
The minimum travel of the needle shield has a length equal to the sum of the lengths corresponding, on the one hand, to the length of the syringe body filled with liquid for injection and, on the other hand, to the length of the needle projecting from the body, to which is added a travel that prevents a finger from touching the end of the needle.
In order to allow the needle shield to move beyond the end of the needle, it is appropriate that after injection, the piston should be free to slide in the syringe body over a length roughly equal to the length of the portion of the needle that projects from the syringe body.
Thus, the syringe body has to be very long, even though only the rear part of the body is actually intended to contain the liquid for injection.
In addition, in order to withdraw liquid, it is necessary for the needle to project into the syringe body. Thus, the needle cannot be interchangeable.
An object of the invention is to provide an injection syringe that makes it possible to reduce the length of the syringe body, thus making the use of standard and interchangeable needles possible.
To this end, a subject of the invention is an injection syringe of the aforementioned type, characterized in that it comprises means of releasing the coupling means to disconnect the actuating plunger and the needle shield during the final phase of the fitting of the needle shield.
According to some particular embodiments, the injection syringe has one or more of the following features:
said release means can be actuated under the control of the needle shield sliding along the syringe body;
it comprises a strong pointer identifying an intermediate position of the shield as it slides along the syringe body, in which position the needle shield is immediately upstream of the release means;
the means of axial coupling comprise means of elastically engaging the needle shield with the actuating plunger (14) and the release means are designed to disengage said means of elastic engagement;
the release means comprise a cam/follower arrangement carried by the needle shield and the syringe body, which arrangement is designed to cause the elastic deformation of the needle shield with a view to disengaging the means of elastic engagement as the needle shield moves;
the needle shield has an operating zone designed to cause the simultaneous movement of the shield and of the actuating plunger along said common portion, which common portion corresponds to the travel of the actuating plunger at the time of injection; and
the coupling means are designed to axially secure the needle shield and the actuating plunger together in both directions over said common portion.